Angiotensin-Converting Enzyme 2 Inhibits Lipopolysaccharide- Caused Lung Fibrosis via Downregulating the Transforming Growth Factor ?-1/Smad2/Smad3 Pathway

Background: In our previous studies, angiotensin-converting enzyme 2 (ACE2) was shown to alleviate the severity of acute lung injury, but its effects on the development of lung injury-caused lung fibrosis have not been studied. Methods: In the present study, the effects of ACE2 on lipopolysaccharide (LPS)-induced fibrosis in the lung were studied. The role of epithelial-mesenchymal transition (EMT) and that of the transforming growth factor #-1 (TGF-#1)/Smad2/Smad3 pathway in LPS-induced fibrosis in the lung were investigated. Results: ACE2 expression in the mouse model of LPS-induced lung fibrosis was significantly increased. ACE2 activator diminazene aceturate (DIZE) significantly reduced pulmonary fibrosis, decreased alpha-smooth muscle actin expression, collagen I, hydroxyproline, and TGF-#1 in the lung. DIZE significantly decreased TGF-#1 expression and the activation of Smad2 and Smad3. ACE2 overexpression inhibited the LPSinduced EMT in MLE-12 cells (lung epithelial cells) and small interfering RNA treatment of ACE2 stimulated EMT. ACE2 overexpression also inhibited TGF-#1 expression and activation of Smad2 and Smad3 in MLE-12 cells. Finally, after MLE-12 cells were treated with both ACE2 and TGF-#1 plasmid, TGF-#1 plasmid significantly abolished the effect of ACE2 plasmid on the EMT in MLE-12 cells. Conclusion: Combined with the in vivo study, it was revealed that ACE2 can suppress the TGF-#1/Smad2/Smad3 pathway in lung type II epithelial cells, thus reversing their EMT and lung fibrosis. The present study provides basic research data for the application of ACE2 in lung injury-caused lung fibrosis treatment and clarifies the intervention mechanism of ACE2 in pulmonary fibrosis, which has potential value for clinical application. SIGNIFICANCE STATEMENT Angiotensin-converting enzyme 2 (ACE2) can inhibit the epithelialmesenchymal transition (EMT) in lung type II epithelial cells and lung fibrosis. ACE2 can regulate the transforming growth factor #-1/Smad2/Smad3 pathway in lung type II epithelial cells, which may be the underlying mechanism of ACE2's effect on EMT and lung fibrosis.